Multi-focal lenses, frequently used for remedying presbyopia, contain two or more regions of differing corrective power. An early attempt at constructing multi-focal lenses involved joining pieces of two separate lenses, one positioned above the other within the eyewires of each of a patient's left and right spectacle frames. Called "Franklin Bifocals," these lenses included distinct horizontal lines demarcating the boundary between the joined lens pieces in each frame. The sharp dividing lines were cosmetically unattractive, however, and the lenses themselves were mechanically weak.
More modern bifocal designs provide far- and near-viewing correction for each eye using a single lens fitted into the eyewire of the corresponding spectacle frame. Typically designed to correct the distance vision of the patient, the single lens includes a cavity or countersink ground into either its front or rear surface. A separate segment button is fused to the countersink to provide the appropriate additive power for near vision correction. Alternatively, the distance prescription may be ground on one surface of a single lens and the power of the addition ground on the other. Each of these designs supplies lenses mechanically stronger than the Franklin bifocal; the designs do not, however, eliminate the unattractive boundary visible between the two prescriptive surfaces. Use of these conventional bifocals also causes patients to experience blurriness in zones corresponding to the demarcation lines as the patients move their eyes vertically.
Progressive power lenses, also known as "invisible" bi- or tri-focal lenses, eliminate the discontinuities visible in other multi-focal designs and resulting vertical blurriness by continuously varying the corrective power throughout particular regions of single lenses. Such progressive power lenses effectively disguise their multi-focal nature by blending adjacent prescriptive curves through grinding and polishing techniques. Blending adjacent curves introduces other optical distortions, however, creating, for some patients, unwanted astigmatism or vertical prism imbalance. Progressive power lenses also typically contain narrow optical corridors connecting the distance and near viewing areas, reducing the peripheral clarity, and resulting comfort level, of many wearers.
An important consideration in accommodating invisible bifocal lenses to the eyes of patients involves orienting the visual axes of the eyes at the optical thresholds where the progressive powers begin. In other words, as the left and right eyes move together vertically in the progressive pathways, they should encounter parts of the invisible bifocal lenses of the same progressive power. This result depends in part on the choice of spectacle frames and whether the respective dimensions of the frames allow accurate centering of the lenses with respect to patients' pupils. Proper alignment also depends on the technique used to determine the centers of the patients' pupils.
A variety of devices exist for assisting a practitioner in objectively determining the centers of a patient's pupils relative to predetermined locations while the patient's head is in its primary position. One such device, the "Multi-Purpose Measuring Device" provided by the Varilux Corporation, is a transparent, lined overlay having a pointed bottom which is designed to adhere temporarily to a patient's spectacle frames. While facing the sitting or standing patient who is wearing spectacles, the practitioner places the pointed end of the measuring device in the deepest point of one of the left or right frames. The practitioner subsequently attempts to align his or her line of sight with the eye of the patient corresponding to the frame containing the measuring device and marks the pupil center on the measuring device using a washable felt tip pen. The practitioner can then read the vertical distance from that pupil center to the frame bottom from the markings on the measuring device. The vertical distance from the center of the patient's other pupil relative to the frame bottom may be determined similarly.
To measure the horizontal distance from the patient's pupil center to a particular location, typically the bridge of the patient's nose, the practitioner orients a measuring device resembling a conventional ruler approximately parallel to the floor and places it slightly above the patient's nose bridge. The practitioner then sights along an imaginary vertical line intersecting the center of one pupil and, using the measuring ruler, determines the horizontal distance from the pupil center to the bridge of the nose. Similarly, the horizontal distance between the pupil center of the other eye and the bridge of the nose may be determined by sighting along an imaginary vertical line intersecting the other pupil.
These measuring techniques lack the precision necessary to position invisible bifocals suitably for many patients. At least a portion of the patient's pupil is obstructed by the lined overlay, decreasing both the patient's ability to focus appropriately and the practitioner's ability to locate the pupil center. Parallax presents another problem, as the practitioner often cannot precisely align his or her line of sight with that of the patient. Because no concrete structure exists against which the practitioner can verify that the visual axis measurements are accurate, errors made in determining the pupil centers cannot be corrected before the prescriptive spectacle lenses are made.
U.S. Pat. No. 4,206,549 to Gould, which patent is incorporated herein in its entirety by this reference, discloses another objective device for determining the centers of a patient's pupils. The Gould device overcomes some of the disadvantages of other techniques by using a transparent plate with a small target mark that may be magnetically clamped to a lens. Initially, a patient is fitted with a pair of spectacle frames, many of which frames are provided to the practitioner with plane simulated lenses in place. If such spectacles lack these "demonstrator" lenses, plane simulated lenses may be formed by cutting a sheet of plastic or similar material and secured in the spectacle frames. The transparent plate subsequently is attached to either the left or right frame (and later to the other frame if necessary) using sets of magnets located on the opposite surfaces of the lens, and the practitioner aligns the target mark with the center of the patient's pupil by sliding the transparent plate across the lens until he or she believes that the target mark is aligned with the patient's visual axis. Although not disclosed in Gould, presumably the frames are removed from the patient's head and a mark is made on the interior surface of the lens corresponding to the location of the target mark.
Once the lens mark is made the plate presumably is removed and a small spot of fluorescent paint is placed on the contra-ocular surface of the simulated lens coincident with the recording mark. The practitioner then aligns himself or herself with the patient and sights the luminous spot to determine if the recording marks are aligned with the center of the pupil. Alternatively, light may be projected on the luminous spot from a position at an angle to the visual axis of the simulated lens. The patient while fixating at infinity views the colored spot and can inform the practitioner whether the mark is properly aligned with the visual axis.
Because the practitioner makes the initial determination of the pupil center in each case, parallax and other misalignment problems--both in sighting along the patient's visual axis and in marking the location of the target mark on the lens surface--remain when using the Gould device. Any bias present in the practitioner's sighting tendencies also affects the resulting measurement, as does any similar predisposition associated with the practitioner's lens marking capabilities. The friction caused by sliding the magnetically clamped plate across the lens surface may cause slight movement of the spectacle frames from their normal position, further decreasing the accuracy of the determination of the relationship between the pupil center and corresponding lens. For patients having long eyelashes or whose spectacle lenses normally are worn close to their eyes, the magnets on the interior surface of the lenses also may contact their eyelashes and cause the patients difficulty in focusing during the examination. Finally, although the patient may confirm the work of the practitioner by noting whether the fluorescent spots align, misalignment does not necessarily provide the practitioner with additional information to increase the probability of proper alignment during the next iteration of the examination.
The Gould patent also discloses a subjective embodiment in which an opaque plate having a pinhole may be magnetically clamped to the lens. The patient may then sight at an appropriately positioned remote light source or target while adjusting the plate until the light source or target is seen through the pinhole. Each pinhole location subsequently is marked, presumably on the interior surface of the lens, by the practitioner after removing the frames from the patient's head. While this embodiment minimizes the effects of parallax when aligning the pinhole and pupil center, it does not diminish the parallax problems associated with marking the location of the pinhole on the lens, nor does it reduce the other obstacles related to use of the objective embodiment of the Gould device. Moreover, because the opaque plate of the subjective embodiment blocks the patient's peripheral vision, binocular fusion cannot occur and phoria may be introduced.